Field
The present specification generally relates to monitoring salt content in process fluids and, more particularly, to monitoring salt content in high resistivity fluids such as crude oil, paraffin oil, gasoline, and diesel fuel.
Technical Background
Salts present in crude oil are preferably removed before or during the refining process. Additionally, monitoring of the salinity of crude oil during the refining process is desirable to determine the effectiveness of the removal process. The amount of salt present in crude oil is a significant factor that may lead to several problems encountered during transporting and refining processes, including fouling, deactivation of catalysts and severe corrosion in pipelines. In essence, presence of salt in crude oil, even at parts per million levels, may cause major damage during the refining process. Thus, a desalting process is a standard operation in the overall refining process of crude oil into various products. Therefore, it is desirable to determine the content of salt in crude oil at, for example, the desalting process in real-time to verify salt is being effectively removed.
The salt content in a given sample of crude oil depends mostly on the source of the crude oil. Specifically, different wells and different types of oil fields exhibit varying salt content. Additionally, the residual salt water in shipment tankers can contaminate the crude oil with salt. In most cases, the salt content of the crude oil consists of salt dissolved in small droplets of water that are dispersed in the crude oil. The chemical composition may also vary, but mostly the sodium chloride is considered as the main source with lesser degree of calcium and magnesium chlorides.
Measurement of salt in crude oil is a major procedure in petroleum industry operations during transportation, refining, processing and production. The need for fast and reliable system of measurement of salt levels is desirable in a desalting process. An analyzer must not only be reliable but must also provide such measurements in adequate time. Existing analyzers developed to determine salt content in crude oil and similar moieties are based on measurement of electrical properties of the crude oil. Specifically, existing analyzers are dependent on conducting electrochemical tests to detect the electroactive species of the analyte (e.g., crude oil) by measuring the conductance of the material. They contain conductive electrodes that are placed in flowing crude oil sample to measure the difference in potential and, thereby, to determine the salinity value. However, such techniques suffer several limitations, including corrosion, precipitation on electrodes, temperature restriction, short lifespan and accuracy of data at low levels (e.g. parts per million). Corrosion may occur due to presence of conductive-metal on both electrodes, and water present in the analyte flow combined with salts. The salt also precipitates on the surface of these electrodes and causes deterioration of the electrode surface, leading to poor durability and reliability of the sensing system. Moreover, conductivity changes when the operating temperature is varied during the analysis, which changes the resulting salinity values in the obtained samples of crude oil.
Efforts have been made to develop a real-time technique to measure salt present in crude oil based on the ASTM methods of crude oil salinity measurement using potentiometric and electrometric techniques. However, the American Standard Test Methods (D 3230 & D 6470) encounter disadvantages and limitations. For example, ASTM D 3230 is an electrometric method that obligates extensive sample preparation with numerous solvents usage such as xylene, butanol, and methyl alcohol. Similarly ASTM D 6470, which is a potentiometric technique, requires extensive preparation. The extensive preparation limits the usability of both methods if fast and immediate results are needed. Robustness, durability, and fast results are desirable for real-time measurement of salt present in crude oil or similar complexes.